BMatthews,
I like the wing tank approach the best. If one were to use the S4233 airfoil at 13.6 % thick, then the whole gallon and a half could be put into four wing tanks about 1-1/2 x 2 x 36 inches long, one in front of the spar and one behind on each wing half.

DT,

It seems to me that the Saito 74 is way too much engine for a model whose span is limited to 10 feet. If you want to keep the 10 foot wing span limitation, you should go to an engine about half the displacement. If you want to stay with the Saito 74, you should consider at least a 14 foot wing span.

Ten foot span option:
Use a Saito 35 or 40. Fuel requirements are cut roughly in half. Empty weight goes to about four pounds, or about 8.5 pounds with a full load of 3/4 of a gallon. With a span of 10 feet and an area of 6-2/3 square feet, the aspect ratio will be 15. The wing loading will be 20.5 ounces per square foot, loaded with fuel. The take off speed is lowered by about 10% and the thrust requirement is decreased by roughly 50%. Twenty % of the thrust requirement decrease comes from a lower wing loading and 25% comes from increased aspect ratio. With the smaller engine, the thrust required for take off is only about one and a half pounds.

Saito 74 option:
To get fuel duration improvement similar to the ten-foot-span option above you would have to go to a 14 foot span. The extra cost to build an airframe of this size would offset most of the cost of buying a smaller engine.